Heretofore, several permanent magnet structures have been designed to provide internal uniform magnetic fields of relatively high magnitude. The following references, which are hereby incorporated herein, describe some of the existing magnetic structures that provide such fields: (1) U.S. Pat. No. 5,216,401, entitled "Magnetic Field Sources Having Non-Distorting Access Ports," issued Jun. 1, 1993, to Leupold; and (2) U.S. Pat. No. 4,837,542, entitled "Hollow Substantially Hemispherical Permanent Magnet High-Field Flux Source For Producing A Uniform High Field," issued Jun. 6, 1989, to Leupold.
These reference patents describe structures composed of permanent magnet shells that form an internal cavity within which a uniform magnetic field is generated. The strength and direction of the internal uniform magnetic field directly depends on the thickness, remanence and direction of magnetization of the permanent magnet shell material. As a result, a variety of different permanent magnet shells, each having its own shape, thickness, remanence and direction of magnetization, have been designed.
For example, in U.S. Pat. No. 5,216,401 issued to Leupold, a magic ring was described. As shown, the magic ring has a permanent magnet shell composed of a plurality of similarly shaped permanent magnet segments that are assembled to form a cylindrical shell with a concentric cylindrical cavity. Although the permanent magnet segments are similarly shaped, they do not have the same direction of magnetization. More specifically, the direction of magnetization of each segment varies as a function of the polar angle around the circumference of the shell. A shell having such a configuration generates a fixed uniform magnetic field within its internal cavity.
One serious drawback to this shell-segment configuration, however, is that the magnitude and direction of the internal magnetic field can not be changed once the shell is constructed. Thus, a separate magic ring must be built to accommodate each application requiring an internal field having a different direction or magnitude. As a result, these prior art structures are not very practical or desirable for many such applications.
One solution to this problem was disclosed in U.S. Pat. No. 4,862,128, entitled "Field Adjustable Transverse Flux Source", issued to Leupold on Aug. 29, 1989, also incorporated herein. Leupold '128 describes a permanent magnet structure wherein the internal working field can be varied continuously from zero to a maximum field. More specifically, the magnetic structure comprises a plurality of hollow substantially cylindrical flux sources embedded within one another such that their longitudinal axis is aligned and the outer radius of the inner structure substantially equals the inner radius of the next structure. The cylindrical flux sources are constructed so that they can be rotated either clockwise or counterclockwise with respect to each other by any desired angle.
Such flux sources are characterized by their ability to add magnetic fields geometrically. Thus, the resultant working field in the center of their cavity is the vector sum of the magnetic fields independently generated by each embedded flux source. Moreover, there is no magnetic field generated external to the outermost cylinder of these structures. As a result, the structure provides an internal magnetic field having a magnitude that can be varied within a predetermined range which directly depends on the size, shape and magnetization of the individual permanent magnet pieces that comprise each embedded cylindrical flux source.
One drawback to this structure, however, is that each embedded cylinder is composed of many permanent magnet segments or wedges, each having a different magnetization. As a result, such cylinders require the assembly of many different permanent magnet pieces. Thus, although flux sources employing this embedded ring structure provide an adjustable internal working field, they are difficult and costly to make.